Wireless networks have become widely distributed nowadays. A widely used type of networks referred to as “wireless LANs” is defined in the IEEE 802.11 standards of which there exists a whole family. One issue in such type of wireless networks is the problem of quality of service. Traditionally the nodes of such kind of networks transmit in the so-called “best effort” mode which means that a sender can transmit at a rate which is currently available at the network. The “best effort” mode does not ensure a certain quality of service (also referred to as QoS) like a minimum assured bandwidth which could enable “real-time” transmission.
However, there have been efforts to implement QoS or real-time transmission in wireless networks. The most relevant known solutions are IEEE 802.11e (see IEEE standard for information technology—specific requirements part 11: Wireless LAN medium access control (MAC) and physical layer (PHY) specifications: Amendment 8: Medium access control (MAC) quality of service enhancements. IEEE Standard 802.11E-2005, 2005) and the Contention-Free Access using the point coordination function (PCF) defined in the IEEE 802.11 standard (see IEEE standard for information technology—LAN/MAN—specific requirements—part 11: Wireless LAN medium access control (MAC) and physical layer (PHY) specifications. IEEE Standard 802.11, 1999/8802-11 (ISO/IEC8802-11:1999), 1999).
In the former, even though different priorities can be assigned to different traffic flows, it still lacks *strict* quality guarantees since the mechanism is contention-based. While real-time packets get assigned higher priority and based thereon have a higher likelihood of getting assigned a short backoff time, still there is the problem that the mechanism is contention based, and if the total number of packets/stations is too high, there is only a statistical chance that the real-time packet is transmitted with the desired quality and there is no strict quality assurance.
On the other hand, the PCF mode in 802.11 can offer strict guarantees. However, mainly due to their complexity, very few implementations can be found in the market. Apart from the drawback of implementation complexity, strict guarantees rely on “medium reservations” that are usually done on a periodic basis, e.g. a given station reserves the channel for transmitting (P bytes) every T seconds. However, this periodicity does not match the real-time traffic pattern from the data sources. Consider for example voice traffic, where the communication channel is typically idle for ⅓ of the time. Several voice codecs tend to optimize the bandwidth use by applying silence suppression functionality, leaving several (periodic) reserved RT slots empty.
One may try to adapt the period of time slot reservation to the voice codec pattern to optimize bandwidth efficiency, or to reduce delays, but never both at one time.
To support Real-time traffic in wireless networks the “traditional” approach is to make periodic time slot reservations at the MAC layer. This is schematically illustrated in FIG. 1A. FIG. 1A shows below the reserved timings for real-time packet transmission indicated as dashs on the time axis, and in the upper part there is shown the packet arrival frequency through dashed boxes, whereas the solid line boxes represent arriving real-time packets. One can see that in case of FIG. 1 there is no significant delay in the transmission of real-time packets, however, the period between two consecutive real-time (RT) time slots is relatively small which increases the likelihood of unused RT time slots, thereby decreasing efficiency.
To increase efficiency one may increase this time period as shown in FIG. 1B. One can see that depending on the arrival time there can be a significant delay in the forwarding of real-time packets, such as in case of the of the fourth real-time packet shown in FIG. 1B which arrives only shortly after the third reserved RT slot and which therefore has to wait for some time until it is sent at the fourth reserved RT slot.
One possibility to decrease the delay would be to decrease the period between two RT slots reserved for real-time transmission as in the example of FIG. 1A, but as previously mentioned this decreases the reservation efficiency because for a given number of real-time packets there will be a higher number of unused RT slots.
Therefore there is a trade-off between short delays and high efficiency with the approach of RT slots granted to particular users and periodically distributed along the time like in time division multiple access (TDMA) systems.
Hence there is a need for an improved mechanism ensuring real-time traffic in wireless networks. Preferably the mechanism should enable strict guarantees to real-time traffic flows while being backward compatible (deployable in existing 802.11 hotspots), and preferably without starving 802.11 flows.